qcacmn: Invalidate ring desc cookie after processing

Currently all the rx ring descriptor contents are left
intact even after these entries are processed. This can,
at times, lead to stale entries being processed, if the
head pointer of any ring is updated before the updated
contents of the ring descriptor gets reflected in the memory.

This can lead to scenarios where the host driver reads a
stale value of sw_cookie, and free/unmap a currently in-use
buffer, thereby leading to the hardware accessing unmapped
memory region.

The sw_cookie is the integral part of al the rx ring
processing. Hence we always mark the sw_cookie as invalid
after dequeuing an entry from the REO2SW ring. Every time
we check for the validity of the sw_cookie before we try to
process an entry from REO2SW ring. if the invalid bit in the
sw_cookie is set, we just skip this entry and move on to the
next entry in the ring.

Change-Id: I0e78fa662b8ba33e64687a4dee4d1a5875ddb4bf
CRs-Fixed: 2730718